A known X-ray generator that generates X-rays by radiating electrons emitted from an electron source onto a target is equipped with an X-ray tube that accommodates the electron source and the target in its sealed interior. A known example of the electron source disposed in the X-ray tube in the related art is a thermal electron source, such as a filament. Some thermal electron sources are of a small type, such as an impregnated hot-cathode electron-emitting device used as an electron source for a cathode-ray tube. An X-ray tube that uses a thermal electron source accelerates part of a thermal electron flux emitted from the thermal electron source that is heated to high temperature to have high energy through a Wehnelt electrode, an extraction electrode, an accelerating electrode, and a lens electrode. At the same time, after a desired shape of electron flux is formed, the shaped electron flux is radiated onto a target formed of metal, such as tungsten, to generate X-rays.
However, when the electron flux accelerated to have high energy is radiated onto the target to generate X-rays, about 1% or less of the energy of the electrons that collide with the target becomes X-rays, and the remaining about 99% or more becomes heat. At that time, the heat generated in the target is radiated as radiant heat, but the heat radiation is sometimes insufficient because the target is in a vacuum. If the amount of heat radiated is small, the target is increased in temperature. Thus, if a target substrate that supports the target is used, the target substrate sometimes melts, and if no target substrate is used, the target sometimes melts. This requires radiating the heat generated in the target quickly.
Japanese Patent Application Laid-Open No. 2002-025792 discloses a technology for enhancing the effect of radiating the heat generated in the target by placing an X-ray tube in the container, filling electrically insulating oil between the container and the X-ray tube, and transferring the heat to the electrically insulating oil.